General Science eBook

In Section 23 we saw that a fall of temperature caused
water vapor to condense or liquefy. If temperature
alone were considered, most gases could not be liquefied,
because the temperature at which the average gas liquefies
is so low as to be out of the range of possibility;
it has been calculated, for example, that a temperature
of 252 deg. C. below zero would have to be obtained
in order to liquefy hydrogen.

Some gases can be easily transformed into liquids
by pressure alone, some gases can be easily transformed
into liquids by cooling alone; on the other hand,
many gases are so difficult to liquefy that both pressure
and low temperature are needed to produce the desired
result. If a gas is cooled and compressed at
the same time, liquefaction occurs much more surely
and easily than though either factor alone were depended
upon. The air which surrounds us, and of whose
existence we are scarcely aware, can be reduced to
the form of a liquid, but the pressure exerted upon
the portion to be liquefied must be thirty-nine times
as great as the atmospheric pressure, and the temperature
must have been reduced to a very low point.

93. Artificial Ice. Ammonia gas is liquefied
by strong pressure and low temperature and is then
allowed to flow into pipes which run through tanks
containing salt water. The reduction of pressure
causes the liquid to evaporate or turn to a gas, and
the fall of temperature which always accompanies evaporation
means a lowering of the temperature of the salt water
to 16 deg. or 18 deg. below zero. But immersed
in the salt water are molds containing pure water,
and since the freezing point of water is 0 deg.
C, the water in the molds freezes and can be drawn
from the mold as solid cakes of ice.

[Illustration: FIG. 56.—­Apparatus
for making artificial ice.]

Ammonia gas is driven by the pump C into the
coil D (Fig. 56) under a pressure strong enough
to liquefy it, the heat generated by this compression
being carried off by cold water which constantly circulates
through B. The liquid ammonia flows through
the regulating valve V into the coil E,
in which the pressure is kept low by the pump C.
The accompanying expansion reduces the temperature
to a very low degree, and the brine which circulates
around the coil E acquires a temperature below
the freezing point of pure water. The cold brine
passes from A to a tank in which are immersed
cans filled with water, and within a short time the
water in the cans is frozen into solid cakes of ice.

CHAPTER IX

INVISIBLE OBJECTS

94. Very Small Objects. We saw in Section
84 that gases have a tendency to expand, but that
they can be compressed by the application of force.
This observation has led scientists to suppose that
substances are composed of very minute particles called
molecules, separated by small spaces called pores;
and that when a gas is condensed, the pores become
smaller, and that when a gas expands, the pores become
larger.